Add customisable shortcut keybinds

[nullprop]

This PR moves most of the shortcut handling from backends to a separate class, and adds ConCommands for binding/unbinding key codes to shortcuts. Should not change default functionality. Adds initial support towards issue #457.

Down the line it would be nice for binds to reside in a separate config file so they can be persistent across sessions, but maybe that should be handled separately from this PR.